This invention relates to a novel thermosetting resin composition comprising a specific modified polycarbodiimide which composition is excellent in solubility in various solvents and storage stability in the form of a solution, the cured product of which is excellent in humidity resistance, transparency, heat resistance, electrical insulation properties, mechanical properties and the like.
Polycarbodiimides are resins obtained by decarboxylation-condensation of polyisocyanate compounds, and their production processes are disclosed in, for example, D. J. Lyman et al., Die Makromol. Chem., 67, 1 (19-63); E. Deyer et al., J. Am. Chem. Soc., 80, 5495 (1958); L. M. Alberion et al., J. Appl. Polym. Sci., 21, 1999 (1977); T. W. Campbell et al., J. Org. Chem., 28, 2069 (1963); Japanese Patent Application Kokai No. 51-61599; and the like.
Polycarbodiimides are such resins that the carbodiimide groups in the molecule can be cross-linked upon heating and their applications as thermosetting resins having excellent heat resistance are expected. However, there is such a problem that in order to sufficiently cure the polycarbodiimide, a long period of time is required even when the polycarbodiimide is heated at a high temperature. And there is another problem that since polycarbodiimides are low in solubility in various solvents and when they are stored in solution, flocculation of polymer proceeds gradually even in a cold dark place, they are difficult to store stably for a long period of time. Thus it cannot be said that the characteristics thereof as a thermosetting resin have been sufficiently utilized in practice.
Therefore, an attempt has been made to improve the curing velocity by mixing various compounds with the polycarbodiimides. For example, Japanese Patent application Kokai No. 61-235414 proposes a thermosetting resin composition obtained by mixing a polycarbodiimide with an active hydrogen-containing compound. However, this method is disadvantageous in that the heat resistance characteristic of the polycarbodiimide is damaged. Also, Japanese Patent Application Kokai No. 4-161426 proposes a thermosetting resin composition obtained by mixing a polyisocyanate compound with a polycarbodiimide. However, the curing temperature of this composition is as high as about 250.degree. C. and hence the curing characteristics-improving effect is still insufficient. In addition, these resin compositions are not improved in storage stability in the form of a solution at all.
Moreover, a thermosetting resin composition which is a mixture of a polycarbodiimide with an epoxy compound and a curing catalyst is known, and, for example, Japanese Patent Application Kokai No. 62-1714 proposes the use of a tertiary amine as the curing catalyst and Japanese Patent Application Kokai No. 4-342713 proposes the use of a zinc halide or a mixture thereof with an onium salt as the curing catalyst. However, among these methods, the former has such a problem that since the tertiary amine has properties of accelerating the cross-linking reaction of polycarbodiimide, the storage stability of the resin composition in the form of a solution is rather deteriorated, and in the latter case, not only is the storage stability of the resin composition unimproved, but also halogens, zinc and the like resulting from the catalyst remain as a migratable impurities, so that the resin composition cannot be used in such fields that said impurities become a problem, in particular, in the electronic parts field, and hence, it follows that the area of use of the resin composition is remarkably restricted.
In short, a conventional thermosetting resin composition comprising a polycarbodiimide as the main component is not always satisfactory in the aspect of balance of collective characteristics of heat resistance, electrical insulation properties, mechanical properties and the like as well as the storage stability in the form of a solution and curing characteristics.